Spectral analysis and density functional theory study of tert-butylhydroquinone

Abstract

Tert-butylhydroquinone (TBHQ) is an important food antioxidant. Based on density functional theory, the B3LYP functional is used to optimize the geometric configuration and calculate the frequency of TBHQ molecule in gas phase at a level of 6-311g (d, p) basis set. On this basis, based on the time-dependent density functional theory, SMD implicit solvent model is selected, and the first 50 excited states of molecule in ethanol solvent are calculated by using B3LYP functional and def2-TZVP basis set. Multiwfn software is used to analyze the vibration of IR spectrum, the influence of interaction among molecules on IR spectrum and the molecular orbital and electron-hole of UV spectrum. Experimentally, Fourier transform infrared spectrometer (FTIR) is used to measure the IR spectrum of TBHQ sample by KBr tablet method. The UV spectrum of the sample determined in the ethanol solvent by ultraviolet visible spectrophotometer. By comparative analysis, it can be seen that the theoretical spectra are in good agreement with the experimental spectra. The characteristic absorption peaks of each group in the IR spectra are obvious, and the theoretical peaks are in good agreement with the positions of the measured peaks. The hydrogen bonding of dimers and polymers in the TBHQ sample can weaken the O—H bond strength of a single molecule, thus weakening the vibration frequency of the O—H bond and resulting in a wide peak at 3670–3070 cm<sup>–1</sup> in the experimental IR spectrum. The UV spectra are mainly formed from the ground state to the first, second, sixth and seventh excited state. The maximum absorption peak in the UV spectrum is below 200 nm and is formed by the transitions of π→π* and σ→π*. The absorption peaks at 268.8 nm and 221.4 nm are formed by the transitions of n→π* and π→π*. It can be seen from the electron-hole distribution diagram that these four excitations are all electron local excitation. This study may play a certain role in understanding the molecular structure and excitation properties of TBHQ, as well as the formation mechanism of its IR and UV spectra, and also conduce to understanding its antioxidant properties.